Sound attenuating gas pipe



p 2 1963 E. LUDLOW ETAL SOUND ATTENUATING GAS PIPE 5 Sheets-Sheet 1 Filed Nov. '7, 1960 INVENTORS. EDMUND LUDL o w AN D BYBENJA Ml/V H. II? WIN QJLLMW a) ArTozNsvs.

Sept. 24, 1963 E. LUDLOW ETAL SOUND ATTENUATING GAS PIPE Filed Nov. 7, 1960 Sheets-Sheet 2 a 'm Z J 3 W 3 ml w\ 0 i \f 1 7 9 ow 2 3 o 3 3 J 8 W 3 INVENTORS Luv/.0 w Bmrp BENJAMIN H IRWIN TTOIZNEYS.

p 2 1963 E. LUDLOW ETAL SOUND ATTENUATING GAS PIPE 5 Sheets-Sheet 3 Filed Nov. 7, 1960 INVENTOR. fomuuo Luo LOW BYAND BENJAMIN H IRwm ATTORNE vs.

p 1963 E. LUDLOW ETAL 3,

SOUND ATTENUATING GAS PIPE Filed Nov. '7, 1960 5 Sheets-Sheet 4 INVENTORQ fomuuo LUDLOW 4 BYBEMTA mm H. I AWIN Arroeuevs.

Sept. 24, 1963 E. LUDLOW ETAL scum ATTENUATING GAS PIPE 5 Sheets-Sheet 5 Filed Nov. 7', 1960 INVENTORSI famous LUDLOW mm ByBswAmm H. Iewm S A'rro Evs,

lll -r 3,104,734 ND ATTENUATING GAS PIPE SUU Edmund Lndiow and Benjamin H. Irwin, Columbus, Ind,

assignors to Arvin Industries, Inc Columbus, End, a corporation of Indiana ll 'iled Nov. 7, 1960, Ser. No. 67,804 24 Claims. (Cl. 131-48) This invention relates to a sound attentuating pipe, and more particularly to a sound attentuating exhaust pipe for use in association with internal combustion en gines for conveying the exhaust gases therefrom and for attenuating the noise level of said exhaust gases.

It is an object of our invention to provide a sound attenuating pipe which will convey the exhaust gases emanating from an internal combustion engine, which will attenuate the noise level of such exhaust gases, and which will effect such sound attenuation over a wide range of frequencies. It is a further object of our invention to provide a pipe having such sound attenuating properties which can be easily and economically manufactured from metal-tubing, which will have limited space requirements, and which will provide an effective thermal insulation for hot exhaust gases.

According to one form of our invention, We

provide a pair of pipes carried one within the other in coaxial alignment. The inner one of said pair of pipes defines the main gas-flow passage throughout its length in the outer pipe, and is provided with a plurality of axially spaced pairs of beads. Each of said pair of beads abuts the inner wall of the outer pipe to hold the inner pipe in a fixed spaced relationship to said outer pipe and to define with an adjacent pair of beads a resonator volume interposed between the two pipes for attenuating the noise level of the exhaust gases passing through the inner pipe.

The beads in each pair of beads are juxtapositioned to provide an arcuate throat for an adjacent resonator volume. One end of said throat constitutes an interruption or opening in the bead immediately adjacent the resonator volume with which the throat is in communication, and the other end of the throat constitutes an opening formed in the inner pipe within the axial extent of said throat. The throat openings in the inner pipe may be disposed at various arcuate distances from the throat openings in the beads so that a plurality of said pairs of beads may be employed to provide a plurality of resonator volumes having different throat lengths'for attenuating the noise level of different sound wave frequencies.

Other objects and features of our invention will become apparent from the more detailed description which follows and from the accompanying drawings, in which:

FIG. 1 is an isometric view' partially in section showing a sound attenuating exhaust pipe embodying our invention;

FlG. 2 is an enlarged fragmentary longitudinal section of the pipe shown in FIG. 1;

FIG. 3 is an enlarged vertical section taken on the line 3-3 of FIG. 2;

FIG. 4 is an enlarged horizontal line 4-4 of FIG. 2;

FIG. 5 is an enlarged vertical section taken oil the line 5-5 of FIG. 2;

FIG. 6 is a fragmentary isometric view of the inner pipe shown in FIG. 2;

FIG. 7 is a'fragmentary longitudinal section fied form of our invention;

FIG. 8 is a fragmentary longitudinal section of another modification of our invention;

section taken on the of a modi- 3 ,104,734 Patented Sept. 24, 1963 other modification of our invention;

FIG. 16 is an enlarged vertical section taken on the line 16-16 of FIG. 15;

FIG. 17 is a fragmentary longitudinal section of another modification of our invention;

FIG. 18 is an enlarged vertical section taken on the line 18-18 of FIG. 17;

FIG. 19' is a fragmentary longitudinal section of another modification of our invention;

FIG. 20 is an enlarged vertical section taken on the line 28-20 of FIG. 19; and

FIG. 21 is a fragmentary isometric view of the inner pipe shown in FIG. 20.

In the operation of a conventional internal combustion engine in an automobile, the combustion of fuel within the cylinders produces a substantial volume of hot exhaust gases which are exhausted with substantial noise into one or more exhaust manifolds in communication with the exhaust ports of the several cylinders. The frequencies of the sound waves in such exhaust gases extend over a wide range, such as for example from about 30 cycles per second to about 5,000 cycles per second, and in many exhaust systems it is the lower range of frequencies, i.e., frequencies below 200 cycles per second, that are the most diflicult to attenuate or silence. This low range of frequencies below 200 cycles per second is the most diflioult to attenuate because the firing frequency of the engine falls within this range as does the natural resonance frequency of the exhaust system itself.

In a conventional automobile exhaust system, these exhaust gases are silenced by a mufiler connected between an exhaust pipe joined to the exhaust manifold and a tail pipe leading from the muffler to a gas discharge point. Such mufiiers conventionally comprise an outer shell hav-' ing an elliptically or circularly shaped cross-section several times larger than the cross-section of the exhaust pipe, and having a relatively small number of tuned resonator chambers adapted to attenuate the noise level of the exhaust gases passing therethrough. In order for the muffler to achieve maximum silencing it is necessary that a pair of these chambers with large volumes be tuned to attenuate the narrow bands of the wave frequencies of the first overtones of the standing waves of the exhaust and tail pipes. All of these resonator chambers in the mufiier are bafiled from one another, and are arranged within the outer muffler shell in staggered patterns which, combined with the large volumes of the chambers for the exhaust and tail pipes, results in the mufiler being rather large and difficult to mount within the limited space available on the underside of an auto mobile.

Our invention is adapted to attenuate the noise level of the exhaust gases over a wide range of frequencies by passing said gases through an exhaust pipe construction containing a series of a relatively large number of small in-line resonators disposed along the length of the exhaust system and attenuating different, and overlapping, bands of wave frequencies. While our invention may effect ;.the outer pipe 10, and

' such an attenuation of the exhaust gas noises alone, if

desired, it may also be used in combination with a conventional muffler, or incorporated within a muffler as an acoustical mufiler component, or in combination with an acoustical liner.

Asshown in FIG. 1, our invention comprises an outer pipe adapted to be connected at one of itsends to an exhaust manifold 12 by a conventional mounting flange 14. Conveniently, the pipe 11 may havethe same outer diameter of about 2.25 inches as is normally used in exhaust pipes and-tailpipes in conventional exhaust sys- .tems. v p

A second. or inner pipe 16 is coaxially carried within the pipe ltt or for only a portion of the length'of the outer pipe 10, depending upon the degree of sound attenuation desired. ,The inner pipe 16 throughout its length constitutes the main gas-flow passage for the exhaust gases moving through the system. As shown in posed between the inner and outer pipes. The side walls of the volumes are formed by the adjacent walls of the inner and outer pipes thus giving said volumes the configuration of extended annuli. As will be apparent, the volumes 19 also serve as thermal insulation barriers for the hot exhaust gases passing through the pipe 10, and they deaden any metallic ring in either of thepipes 10 or 16.

As shown in FIG. 2, the juxtapositioned walls of each pair of beads 17 and 18 form an arcuate volume throat 20 extending around the outer face of the inner pipe 16 The ends of each of the throats 20 are formed by an opening 22 cut in the pipe 16 between each pair of beads 17 and 18 and an interruption or opening 23 formed in the bead 17 and arcuately spaced around the pipe 16 with respect to the opening 22. Inthis manner, each of the resonator volumes 19 is in operative communication with the main gas-flow passage through the inner pipe 16 10 1" attenuating the noise level of the exhaust gases passing therethrough. V 7 It is well established in the field or resonator design that a volume employed in combination with a relatively long throat Will attenuate a lower range offreque'ncies than if it is employed with a relatively short throat. It

may extend either the length of' fOf course, the dam 24 and the adjacent pipe opening 22 7 t shown in FIG. 5, gives the resonator throat a length substantially-equal to the circumference of the inner pipe 16 for thus attenuating the lower sound wave frequencies.

may be disposed at any desired arcuate distance from the bead opening 23, but by locating them in the positions plurality'of axially spaced pairs of inwardly pro ecting beads 27 and 28 which bear against and support an inner V illustrated in FIGS. 2-and 3.

shown in FIG. 5, the maximum throat length is obtained.

Movement of the opening 22, or the opening 22 and the dam 24, in a clockwise direction in FIG. 5 will, of course, shorten the throat length for attenuating the noise level of the higher sound Wave frequencies.

The same principles of operation are employed in the modification illustrated in FIGS. 7 and 10 as those However, in the modification of FIG. 7 the outer pipe 26 is provided with a pipe 29 coaxially carried within the outer pipe and providing the main gas-flow passage for the exhaust gases throughout its length within said outer pipe. As shown in FIG. 7, the beads 27 and 28 in each pair of beads form the adjacent end wallsof a pair of adjacent'resonator volumes 31 the side walls of said volumes being formed by the adjacent walls of the inner and outer pipes.

As shown, the'section of the outer pipe 26 between the juxtapositioned walls of the beads 27 and 28ccoperates with said walls to form an elongated arcuate throat 31 extending around the outer face of the inner pipe 29. The ends of said throat are defined by an open- 7 ing 32 formed in the inner pipe 29 between the beads is therefore desirable that at least a portion of the volumes 19 be in operative communication with relatively long ,throats 20 for attenuating the noise level of the wave .frequencies below 200 cycles per second. 'In the embodiment shown in FIG. 3, the maximum throat length obtainable is equal to one-half of the circumference of the inner pipe 16. If the opening 22 is located in the pipe 16' at a position other than 180 from the opening 23,

two throats of unequal length will be formed which act in combination with each other and an adjacent volume I to produce, in effect, a single throat for the volume in As shown in FIG. 5, the length of the throat 20 may. be increased to a distance substantially equal to the cir-' cumference of the pipe 16 by forming the opening 22 in the pipe 16 adjacent the opening 23 in the bead 117 and interposing a dam 24 between said openings block- 24, which may be in the form of a weld fillet or a drawn section of the pipe 16, extends between the adjacent walls of the inner and outer pipes 16 and 10 and thus, as

' ting the space between'the beads 17 and 18. The dam 27 and 28 and an interruption or opening 33 formed in the bead 28. In this manner, each of. the resonator volumes 31) is disposed in operative communication with the main gas-flow passage of the inner pipe 29 by one,

of thearcuate throats 31 for attenuating the noise level of the gases passing through said pipe.

In the modificationillustrated in FIG. 8, there is provide an outer pipe 34 having an inner pipe 35 supported within it and constituting the main gas-flow passage throughout itslength within said outer pipe. The inner pipe 35 is provided with a pair of axially spaced out- .wardly projecting beads 36 extending circumferentially around the pipe 35 and engaging the inner walls of the pipe 34 to thus support the inner pipe stretches between the beads in. spaced relationship to said outer pipe and to form the opposed end walls of a pair of adjacent resonsator volumes 37. Each pair of adjacent resonsators and their throats have a common end wall formed by a cncumferentially extending, outwardly projecting bead 38 formed on the inner pipe 35 between each pair of adjacent beads 36 and engaging the inner wall of the outer pipe 34. v Interposed between the bead 38 and each of the beads 36: is a third head 39 formed in the inner pipe 35 and engaging the outer pipe 34. Thejuxtapositioned walls of the beads 39 and the bead 38 form pair of arcuate volume throats 40 and 41 extending around the outer face of the inner pipe 35. The ends of each of the throats 40 and .41 are formed by a pair of openings 42 formed in the inner pipe 35 on either side of the bead 38 between the beads 39 and a pair of openings 43 formed in the beads 39. In the embodiment shown in FIG. 8, the throat 40 shown to the left corresponds to the volunie throat shown in FIG. 3, and the throat 41 shown to the right corresponds to the volume throat shown in FIG. 5, the throat 41 being provided with a dam 44 interconnecting the right hand bead 39 and the bead 38 to thus give the throat 41 a longer length than the throat 40 so that its adjacent volume 37 will attenuate a lower range of frequencies.

The modification shown in FIG. 9 also comprises an outer pipe 45 having an inner pipe 46 coaxially carried within it and constituting the main'gas-fiow passage for the exhaust gases throughout'its length in the pipe 45. The pipe 46 has a plurality ofaxially spaced helical beads 47 projecting outwardly therefrom to engage the inner walls of the outer pipe 45 and support the stretches of the pipe 46 between said beads in spaced relation to the outer pipe 45. As shown in FIG. 9, the end of each of the helical beads 47 shown to the left has a closed turn so that each pair of adjacent helixes form the end walls of an axially extending resonator volume 48 interposed between the inner and outer pipes 46 and 45. The other turns of each of the helixes 47 are axially spaced along the inner pipe 46 with the extreme end turn of each of said helixes being open, as at 49, said opening constituting the volume-end of the elongated throat formed by the adjacent walls of the several helical turns of the bead 47. The opposite end of said throat is formed by an opening 50 formed in the inner pipe 46 between the closed end of the helix and the first turn thereof. As will be understood, this helical bead construction provides a resonator volume throat having a length equal to several times the circumference of the inner pipe 46, and is thus well adapted for operating upon extremely low sound wave frequencies.

In the modification shown in FIGS. 11 and 12, there is provided an outer pipe 52 having an inner pipe 53 supported within it and constituting the main gas-flow passage throughout its length within said outer pipe. The inner pipe 53 is supported in spaced relationship to the pipe 52 by a plurality of pairs of annular flanges 54 and 55 fixedly secured to the outer walls of the inner pipe 53 and bearing against the inner walls of the outer pipe 52. As shown in FIG. 11, each of the flanges has a Z-shaped cross-section with the walls on the adjacent pairs of flanges 54 and 55 normal to the pipe axes forming the end walls of elongated resonator volumes 56 interposed between the inner and outer pipes 53 and 52. As shown in FIG. 11, the legs on each of the flanges 54 and 55 are oriented in the same direction to facilitate insertion of the pipe 53 with the flanges mounted thereon into the outer pipe 52.

The juxtapositioned walls of each pair of flanges 54 and 55 normal to the pipe axes form an arcuate volume throat 57 extending around the outer face of the inner pipe 53. The ends of said throat 57 are formed by an opening 58 formed in the wall of the flange 55 normal to the pipe axes, and by a pair of aligned openings 59 formed in the inner pipe 53 and the axially extending leg of the flange 54 abutting the outer wall of said inner pipe. The openings 59 and 58 in each of the throats 57 are arcuately spaced around the inner pipe 53 to thus give said throats an elongated arcuate configuration. In this manner, each of the resonator volumes 56 is disposed in operative communication with the main gas-flow passage of the pipe 53 by means of one of the throats 57 for attenuating the noise level of the exhaust gases passing through said pipe.

In the modification shown in FIGS. 13 and 14, there is provided an outer pipe 61 having an inner pipe 62 supported within it and constituting the main gas- W passage throughout its length within the outer pipe. The inner pipe 62 is supported in spaced relation to the pipe 61 by a plurality of axially spaced annular rings 64 hearing against the opposed walls of the inner and outer pipes 62 and 61. As shown in FIG. 13, each of the rings 64 has an outwardly open channel section against the outer face of the inner pipe 62, and whose legs are integrally connected to a pair of axially extending, annular flanges 66 bearing against the inner Wall of the outer pipe 61. As shown, the adjacent legs of adjacent ring channels 65 form the end walls of a plurality of axially extending'resonator volumes 67 interposed between the inner and outer pipes 62 and 61.

The juxtapositioned legs on each ring channel 65 are normal to the pipe axes and cooperate with the section of the outer pipe overlying said channel to form an arcuate volume throat 68 extending around the inner face of the outer pipe 6-1. The ends of each throat 68 are formed by an opening 69 formed in one of the ring channel legs, and

whose Web is disposed 6. by a pair of aligned openings 70 formed in the inner pipe 62 and the ring channel web abutting the outer wall of said inner pipe. As shown in FIG. 14, the openings 69 and 70 in each of the throats 68 are arcuately spacedv around the rings 64 to thus give said throats an elongatedarcuate configuration. In this manner, each of the resonator volumes 67 is disposed in operative communication with the main gas-flow passage of the pipe 62 by means of one of the throats 68 for attenuating the noise level of the exhaust gases passing through said pipe.

In the modification illustrated in FIGS. 15 and 16, there is again provided an outer pipe 72 having an inner pipe 73 supported within it and constituting the main gas-flow passage throughout its length within the outer pipe. The outer pipe 72 has a plurality of inwardly projecting and circumferentially extending beads 74 which engage the inner pipe 73 and support it in spaced relation to the outer pipe stretches between said beads. As shown in FIG. 15, the beads 74 are axially spaced along the pipe 72, and each of said beads comprises an annular web 75 bearing against the outer face of the pipe 73 and integrally joined to a pair of juxtapositioned Walls 76- normal to the pipe axes and integral with the outer pipe 72. The adjacent walls 76 on adjacent beads 74 form the end walls of a plurality of axially extending resonator volumes 77 interposed between the inner and outer pipes 73- and 72.

As shown in FIG. 15, a plurality of annularly extending rings 78 are rigidly secured around the outer pipe 72 in positions to overlie each of the beads 74. In this manner, the juxtapositioned walls 76 of each of the beads 74 cooperate with the rings 78 to form a plurality of axially spaced arcuate volume throats 79 extending around the outer face of the inner pipe 73. The ends of each throat 79 are formed by an opening 80 formed in one of the walls 76 of the head 74, and by a pair of aligned openings 81 formed in the inner pipe 73 and the web 75 of the head 74. As shown in FIG. 16, the openings 80 and 81 in the throats 79 are arcuately spaced around the beads 7-4 to thus give said throats an elongated arcuate con figuration, and to dispose each of the resonator volumes 77 in operative communication with the main gas-flow passage 73 for attenuating the noise level of the exhaust gases passing through said pipe.

In the modification shown in FIGS. 17 and 18, there is again provided an outer pipe 32 having an inner pipe 83 supported within it and constituting the main gas-flow passage throughout its length along the outer pipe. The inner pipe 83 is supported in spaced relationship to the pipe 82 by a plurality of axially spaced annular rings 84 fixedly secured to the outer walls of the pipe 83 and hearing against the inner walls of the outer pipe 82. As shown in FIG. 17, each of the rings 34 comprises a centrally disposed outwardly projecting bead 85 whose juxtapositioned Walls generally normal to the pipe axes are integrally connected to a pair of annularly extending flanges 86 fixedly mounted on the pipe 83. As shown, the adjacent Walls on adjacent pairs of ring beads 85 form the end walls of a plurality of axially extending resonator volumes 87 interposed between the inner and outer pipes 83 and 82.

The juxtapositioned walls on the beads 85 cooperate with the outer face of the inner pipe underlying said beads to form a plurality of axially spaced arcuate volume throats 88 extending around the outer face of said pipe. The ends of each throat are formed by an opening 89 formed in one of the walls of the ring bead 85, and by an opening 90 formed in the inner pipe 83 within the axial extent of the head 85. As shown in FIG. 18, the openings 89 and 90- in the threats 88 are arcuately spaced around the rings 84 to give said throat an arcuate configuration, and to dispose the resonator volumes 87 in operative communication with the main gas-flow passage of the pipe 83 for attenuating the noise level of the exhaust gases passing through said pipe.

In the modification illustrated in FIGS. 19-21, there is provided an outer pipe 92 having an inner pipe 94 supto form the end walls of a plurality of axially spaced resonator volumes 96, the side walls of which are formed by the adjacent walls of the inner and outer pipes.

As shown in FIG. l9, each of the projections 95. has a substantialaxial extent, and is provided with an inward ly projecting bead 97 extending inwardly. from one end of said projection but terminating short of the oppo- 7 site end of said projection. The walls of the projection I 95 along the edges of said bead bare against the inner Walls of the outer pipe'92 so that said bead acts in combination with the portion of the outer pipe 92 overlying it to form an elongated volume throat. The ends of each of the throats are formed by the open end of the bead in communication with its adjacent resonator volume 96 and by an'opening 98 formed in said bead adjacent the opposite end thereof. As is shown in FIG. 21, with each of the beads 97 extending inwardly from only one edge of its respective projection 95, each of the volume throats will be disposed in operative communication with only one of the resonator volumes 96.

' Alternatively, the inner pipe 94 may constitute a plus rality of individual pipe segments, in which case each of said segments may comprise that portion of the inner pipe disposed between the dotted lines A--A' in FIG. 21,

or between the dotted lines B-A' in said FIG. 21. In

the former case, the adjacent ends of adjacent segments are disposed in abutting engagement within the outer pipe 92 and the opening from the main gas-flow passage into each throat constitutes the opening 98, while in the latter case each throat has both of its ends open and the adjacent ends of adjacent segments are spacedslight-ly apart, said spacing forming the opening from themain gas-flow passage into the throats. v

When our invention is not used as the sole means for silencing the exhaust gases of an engine, such as for ex ample when it is used in combinationwith a conventional mufiler or as a component of a mufiler, short sections of the pipes containing the in-line resonators are produced.

These short sections may then be employed assections of the exhaust pipes interconnecting the manifolds and muffiers, or used as components of conventional mothers,

in which case they are merely mounted within the muffler shells.

For purposes of simplicity of description we have only described our invention for use in an exhaust system. However, it may, of course, also be used on the intake side ofan internal combustion engine for transporting and silencing the gaseous mixtures into the engine, or for any other applications involving the'mcvement and silencing of gases.

We claim as our invention: 7

1. In a sound attenuating gas pipe for conveying and attenuating the noise level of a moving gas stream, an outer pipe, an inner pipe carried within saidouter pipe and providing a main axially extending gas-flow passage throughout its length in said outer pipe, and means axially spaced along at least one of said pipes and extending circumferentially therearound to abut the-adjacent wall of the other pipe to form the end walls of a plurality of axially spaced, resonator volumes interposed between the adjacent pipe walls, said means having pluralities of juxtapositioned wall surfaces which cooperate with the 8 V openings in'said inner pipe being within the axial extent of one of said throats.

2; The invention as set forth in claim 1 in which said juxtapositioned wall surfaces are arcuate to form a plurality of arcuate volume throats, and said openings in said means and inner pipe are arcuately spaced from each other.

3. in a sound attenu atin as i i e for conveying and attenuating the noise level of a moving gas stream, an

' outer pipe, an inner pipe carried within said outer pipe and providing a main gas-flow passage throughout its length in said outer pipe, and circumferentially extending means axially spaced along the gas pipe operatively interconnecting the adjacent walls of the inner and outer pipes to define a plurality of axially spaced resonator volumes interposed between said pipes and a plurality of arcuate open-ended volume throats, the ends of each of said throats being formed by an opening formed in the inner pipe adjacent each volume and an opening formed in said means opening into each volume and arcuately spaced from the opening in said inner pipe.

4. The invention as set forth in claim 1 in which said means comprises a plurality of axially spaced beads formed in one of said pipes and abutting the adjacent Wall of the other pipe, the walls of said beads cooperating with the adjacent walls of the inner and outer pipes to form said plurality of resonating chamber volumes and said plurality of volume threats.

5. The invention as set forth in ciaim 1 in which said means comprises a plurality of pairs of beads formed on one of said pipes and abutting the adjacent Walls of the other'pipe, said. pairs of 'beads and the adjacent walls of the inner and outer pipes forming said plurality of volumes and throats, one of the beadsin each pair of beads extending circumferentially around said one pipe and the other bead in each of said pair of beads having a circumferentially disposed opening to form said opening in said means, each of the openings in said inner pipe being interposed between the'beads in each pair of beads.

6. The invention as set forth in claim 5 in which said pairs of beads are formed in said one of said pipes with said one and said other of the beads being alternately arranged along the lengths of said pipes. 7. The'invention as set forth in claim 5 with the addition that the beads in at least one. of said pairs of beads are interconnected by a dam interposed between the openings in said other bead and the inner pipe to provide a unidirectional arcuate flow through the throat formed by said pair of beads.

8. The invention as set forth in claim 7 in which said dam is immediately adjacent the opening in said other head and the opening in said inner pipe is immediately adjacent said dam to form a throat having a length substantially equal to the circumference of the inner pipe.

9. The invention as set forth in claim 1 in which said means comprises a first pair of axially spaced circumferentia'lly extending beads formed in one of said pipes and forming the remote end Walls ofa pair of said volumes, a circumferentially extending third beadformed in one of said pipes and interposed between said first pair of beads, and a second pair of beads formed in one of said pipes on opposite sides of said third head to form therewith a pair of arcuate throats for said'volumes and forming the adjacent end'walls of said volumes, sm'd second pair of beads extending only substantially circumferentially around said inner pipe to form said openings in said means, and said openings in said inner pipe being interposed between said third bead and each of the beads in said second pair of beads. a 10. The invention as set forth in claim 9 With the addition that a dam is interposed between said third beadand at least one of the beads in said second pair of beads to 11. The invention as set forth in claim 1 in which said means comprises a plurality of axially spaced helical beads formed in one of said pipes and having a plurality of axially spaced turn-s abutting the adjacent wall of the other pipe, one end of each helix having a closed turn and the opposite end having an open turn to form the opening in said means, and said openings in the inner pipe are interposed between a pair of adjacent turns in each of said helixes.

'12. The invention as set forth in claim 1 in which said means comprises pairs of annular axially spaced flanged members interconnecting the adjacent walls of said inner and outer pipes, each of said pairs of members having a pair of annular walls normal to the pipe axes whereby said pairs of annular walls and the adjacent walls of the inner and outer pipes form said plurality of volumes and throats, the openings in said means and inner pipe comprising an opening formed in one of the walls in each pair of members and an opening in said inner pipe between the wall-s in each pair of members opening into the space between each pair of said walls.

13. The invention .as set forth in claim 12 in which each of said members has a zhaped cross-section and each of said annular walls forms a web interconnecting a pair of axially extending flanges bearing against the adjacent walls of said inner and outer pipes, the flange on one of said pair of members engaging the inner pipe having an opening formed therein in alignment with the opening in said inner pipe.

14. The invention as set forth in claim 13 in which the flanges engaging the inner pipe are all oriented in one axial direction and the flanges engaging the outer pipe are all oriented in an opposite axial direction.

15. The invention as set forth in claim 1 in which said means comprises a plurality of axially spaced annular rings, each of said rings having an annularly extending channel interposed between said inner and outer pipes whereby said rings and the adjacent walls of said inner and outer pipes form said plurality of volumes and throats.

16. The invention as set forth in claim 1 in which said means comprises a plurality of axially spaced annular rings interposed between the inner and outer pipes, each of said rings having an outwardly open channel comprising a web bearing against the inner pipe and a pair of annular legs interconnecting the inner and outer pipes, the openings in said means and inner pipe comprising openings formed in said webs and one of the legs in said pairs of legs, and openings formed in the inner pipe in alignment with said web openings.

17. The invention as set forth in claim 16 in which each of said rings has a pair of annular flanges projecting outwardly from said pair of legs .and engaging the inner wall of said outer pipe.

18. The invention as set forth in claim 1 in which said means comprises a plurality of inwardly projecting beads formed on said outer pipe and engaging said inner pipe, and a plurality of annular rings disposed around said outer pipe over said beads, said openings in said means and inner pipe comprising openings formed in one of the side walls of said beads and a pair of aligned openings formed in the inner pipe and the portions of the beads engaging the inner pipe.

19. The invention as set forth in claim 1 in which said means comprises a plurality of axially spaced rings interposed between the inner and outer pipes, each of said lid rings having a circumfercntially extending inwardly open channel abutting the adjacent walls of the inner and outer pipes, and said openings in said means and inner pipe comprise an opening formed in a side wall of each of said beads and an opening formed in the inner pipe within the axial extent of each of said beads.

20. The invention as set forth in claim 19 with the addition that each of said rings has a pair of annular flanges projecting axially outwardly from the opposed sides of said channel and bearing against the inner pipe.

21. The invention as set forth in claim 1 in which said means comprises a plurality of elongated projections formed in one of said pipes and abutting the other of said pipes, each of said projections having an elongated bead formed therein and cooperating with the walls of said other pipe to form said plurality of throats, and said openings in said means and inner pipe comprising an open end formed in each of said beads in communication with one of said volumes and openings formed in said inner pipe within the axial and radial extent of said beads.

22. The invention as set forth in claim 1 in which said means comprises a plurality of elongated projections formed on said inner pipe and abutting said outer pipe, each of said projections having an elongated bead formed therein and cooperating with the walls of said outer pipe to form said plurality of throats, and said openings in said means and inner pipe comprising an open end formed in each of said beads in communication with one of said volumes and openings formed in said inner pipe within the axial and radial extent of said beads.

23. The invention as set forth in claim 1 in which said inner pipe comprises a plurality of pipe segments having their adjacent ends spaced closely together, and said means comprises elongated projections formed on the opposed ends of said segments and abutting the walls of the outer pipe, one of the projections on each segment having an elongated open-ended bead formed therein and cooperating with the overlying walls of the outer pipe to form one of said plurality of throats the ends of which are formed by the open ends of said head opening into one of said volumes and the spacing between the pipe sections.

24. The invention as set forth in claim 1 in which said inner pipe comprises a plurality of pipe segments the ends of which are in abutting engagement, and said means comprises elongated projections formed in the opposed ends of said segments and abutting the walls of the outer pipe, one of the projections on each segment having an elongated bead formed therein and cooperating with the walls of the outer pipe to form one of said plurality of throats, each of said beads having one of its ends opening into one of said volumes and an opening adjacent its opposite end opening into said main gas-flow passage.

References Cited in the file of this patent UNITED STATES PATENTS 2,099,858 MacKenzie et al Nov. 23, 1937 2,188,202 MacKenzie et a1. Jan. 23, 1940 2,357,792 Powers Sept. 5, 1944 2,632,521 Eaton Mar. 24, 1953 2,643,731 Sohmid June 30, 1953 FOREIGN PATENTS 334,717 Great Britain Sept. 11, 1930 638,787 Germany Nov. 23, 1936 

1. IN A SOUND ATTENUATING GAS PIPE FOR CONVEYING AND ATTENUATING THE NOISE LEVEL OF A MOVING GAS STREAM, AN OUTER PIPE, AN INNER PIPE CARRIED WITHIN SAID OUTER PIPE AND PRIVIDING A MAIN AXIALLY EXTENDING GAS-FLOW PASSAGE THROUGHOUT ITS LENGTH IN SAID OUTER PIPE, AND MEANS AXIALLY SPACED ALONG AT LEAST ONE OF SAID PIPES AND EXTENDING CIRCUMFERENTIALLY THEREAROUND TO ABUT THE ADJACENT WALL OF THE OTHER PIPE TO FORM THE END WALLS IF A PLURALITY OF AXIALLY SPACED RESONATOR VOLUMES INTERPOSED BETWEEN THE 